Relocate StackableStreambuf to IOStreams.

IOStreams/StackableStreambuf.h

@@ -0,0 +1,69 @@
+static_assert( __cplusplus > 2020'99 );
+
+#pragma once
+
+#include <Alepha/Alepha.h>
+
+#include <ostream>
+#include <exception>
+#include <stdexcept>
+#include <memory>
+#include <stack>
+
+namespace Alepha::Hydrogen::IOStreams  ::detail::  stackable_streambuf
+{
+	inline namespace exports
+	{
+		struct StackableStreambuf;
+
+		template< typename > struct PushStack;
+
+		struct PopStack{};
+	}
+
+	struct exports::StackableStreambuf
+		: virtual public std::streambuf
+	{
+		public:
+			std::streambuf *underlying;
+
+			~StackableStreambuf() override;
+
+			// Children must be created by `new`.
+			explicit StackableStreambuf( std::ostream &host );
+
+			auto out() const { return std::ostream{ underlying }; }
+
+			virtual void writeChar( char ch )= 0;
+			virtual void drain()= 0;
+
+			int overflow( const int ch ) override;
+
+			std::streamsize xsputn( const char *data, std::streamsize amt ) override;
+	};
+
+	template< typename T >
+	struct exports::PushStack
+		: T
+	{
+		using T::T;
+	};
+
+	template< typename T >
+	std::ostream &
+	operator << ( std::ostream &os, PushStack< T > &&params )
+	{
+		build_streambuf( os, std::move( params ) );
+		return os;
+	}
+
+	inline namespace impl
+	{
+		std::ostream & operator << ( std::ostream &os, PopStack );
+	}
+}
+
+namespace Alepha::Hydrogen::IOStreams::inline exports::inline stackable_streambuf
+{
+	using namespace detail::stackable_streambuf::exports;
+}